In the past, numerous adhesive systems have been proposed and used to achieve such bonding. For example, use has been made of neoprene-phenolic type contact adhesives. Unfortunately, however, the wetting quality of this type of adhesive is unsatisfactory, particularly in the case of EPDM materials. This frequently results in an inability to achieve a continuous adhesive bonding film on the EPDM surface, producing consequential reduction in bond strength. Furthermore, the neoprene-phenolic adhesives do not have the ability to satisfactorily crosslink, greatly reducing their cohesive strength.
Attempts have been made to overcome the wetting problems attendant to this type of adhesive system by using primers to wet the surface of the EPDM films, and thus allow the neoprene-phenolic adhesive to form a continuous bonding film on the primed EPDM surface. While this expedient tends to overcome the wetting difficulty, the lock of strength imparted by good crosslinking still remains a problem. In addition, the application of a primer is labor intensive; consequently, it is frequently desirable to avoid this step. Also the primer must be allowed to sit for an appreciable interval of time after its application, lengthening the installation process and increasing its cost.
One-coat butyl rubber adhesive systems have also been widely used; however, from the standpoint of bond strengths, these materials produce indifferent results, intermediate between the primed and unprimed neoprene-phenolic systems.